Brake systems using compressed air as the transmission medium are widely popular. In these systems, a multiple-circuit brake valve is in most cases actuated by the brake pedal. The pressure is proportional to the force exerted on the brake pedal and is transmitted to so-called boosters or preload cylinders. It is customary to utilize one preload cylinder per brake circuit or also per axle. Such an arrangement consists, for example, of two brake valves and two preload cylinders and can be combined into one unit. In a two-circuit brake unit, the hydraulically and compressed-air actuated pistons are connected in series in a so-called tandem arrangement.
In these systems, anti-lock devices are also utilized. In systems having a so-called separate structure, i.e., a structure where the brake valve and the pressure cylinder are not spatially combined, one pressure cylinder is used for each control conduit. The anti-lock servo member is inserted in the connecting line to the brake valve. The servo member is actuated by an electronic circuit and serves to regulate the pressure in the pressure cylinder. The corresponding pressure level in the hydraulic brake circuit corresponds to the pressure transmitted.
The above described methods for solving the problem in the art represent a not inconsiderable expenditure in apparatus technology, especially when considering the additional measure of providing antilock means. The pressure cylinder combination must be of a correspondingly large dimension so that an appropriate brake pressure is produced even in case of poorly adjusted brakes. These systems, however, are disadvantageous if the corresponding brake circuit exhibits poor air-bleed. In such a case, the full brake pressure will, in most instances, no longer be attained. Without an additional warning means, the driver cannot be made aware of this impairment in functioning. Since most of the braking operations take place in the partial braking zone, the faulty functioning becomes noticeable only in case of full braking due to the lower braking effect, which can then represent a dangerous situation.